Process to synthesize AB-PBO monomer and phosphate salts thereof

ABSTRACT

AB-polybenzoxazole monomer, such as 3-amino-4-hydroxybenzoic acid, can be synthesized in high yields from a hydroxy-benzoic acid or related compound in a three-step process of (1) nitration, (2) hydrolysis of the ester, and (3) reduction of the nitro moiety. The monomer is conveniently recovered as a phosphate salt in high purity by precipitating and recrystallizing from a phosphoric acid solution.

BACKGROUND OF THE INVENTION

The present invention relates to AB-polybenzoxazole (AB-PBO) monomersand processes for synthesizing them.

AB-polybenzoxazole monomers comprise:

(1) an aromatic group;

(2) an o-amino-hydroxy moiety bonded to said aromatic group, whichconsists of a primary amine group bonded to said aromatic group and ahydroxy group bonded to said aromatic group in a position ortho to saidprimary amine group: and

(3) an electron-deficient carbon group linked to said aromatic group.

AB-polybenzoxazole monomers preferably conform with formula I: ##STR1##wherein Q is an electron-deficient carbon group: Ar is an aromaticgroup: and the amine and hydroxy groups are in ortho position withrespect to each other.

The monomer is polymerized by polycondensation in a non-oxidizingsolvent acid, such as methanesulfonic acid or polyphosphoric acid, atelevated temperatures, as described in Sybert et al., Liquid CrystallinePolymer Compositions, Process and Products, U.S. Pat. No. 4,772,678(Sept. 20, 1988): Wolfe et al., Liquid Crystalline Polymer Compositions,Process and Products, U.S. Pat. No. 4,703,103 (Oct. 27, 1987): Wolfe etal., Liquid Crystalline Polymer Compositions, Process and Products, U.SPat. No. 4,533,692 (Aug. 6, 1985): Wolfe et al., Liquid CrystallinePoly(2,6-Benzothiazole) Compositions, Process and Products, U.S. Pat.No. 4,533,724 (Aug. 6, 1985); Wolfe, Liquid Crystalline PolymerCompositions, Process and Products, U.S. Pat. No. 4,533,693 (Aug. 6,1985): Evers, Thermoxidatively Stable Articulated p-Benzobisoxazole andp-Benzobisthiazole Polymers, U.S. Pat. No. 4,359,567 (Nov. 16, 1982);Tsai et al., Method for Making Heterocyclic Block Copolymer, U.S. Pat.No. 4,578,432 (Mar. 25, 1986) and 11 Ency. Poly. Sci. & Eng.,Polybenzothiazoles and Polybenzoxazoles, 601 (J. Wiley & Sons 1988).

The resulting polymers comprise a plurality of mer units which eachcontain:

(1) an aromatic group: and

(2) an oxazole ring fused to said aromatic group and linked at the2-carbon to an aromatic group in an adjacent mer unit.

The polymers preferably comprise a moiety which conforms to formula II##STR2## wherein n is a number of repeating units in excess of one.

The o-amino-hydroxy moiety of the AB-PBO monomer is extremely sensitiveto air oxidation as a free base, so the monomer is ordinarily stored asan acid salt of hydrogen chloride. The release of hydrogen chloride gasduring polymerization causes foaming of the polymerization mixture whichcan interfere with the reaction. Typically, the monomer isdehydrohalogenated at moderate temperatures and, optionally, reducedpressure, in a solution of polyphosphoric acid having low viscosity, andthen phosphorus pentoxide is added to the solution before polymerizingto high molecular weight, as described in U.S. Pat. No. 4,533,693 whichis previously incorporated by reference. The dehydrohalogenation steprequires time and requires that equipment be made of materials which areinert with respect to hydrogen halide gases. What is needed is a monomerwhich does not require a dehydrohalogenation step.

The condensation of AB-PBO is a polycondensation reaction. In such areaction, the purity of the resulting monomer is critical, becauseimpurities containing only one reactive group act as chain-terminatingagents which hold down the molecular weight of the resulting polymer.What is needed is a high yield process to synthesize AB-PBO monomerhaving a high purity.

SUMMARY OF THE INVENTION

One aspect of the present invention is a process for synthesizing anAB-PBO monomer, said process comprising the steps of:

(1) contacting a hydroxy-ester compound comprising:

(a) an aromatic group:

(b) a hydroxy group bonded to said aromatic group: and

(c) an ester moiety having a carboxylate moiety linked to said aromaticgroup

with a nitrating agent under conditions such that the aromatic group isnitrated in a position ortho to the hydroxy group, in an organicsolvent, which is inert with respect to all reagents under reactionconditions:

(2) converting the nitrated hydroxy-ester of step (1) to a water-solublenitrated hydroxy-benzoate salt and dissolving said water-soluble salt inan aqueous solvent: and

(3) contacting the water-soluble salt product of step (2) with ahydrogenating agent in the presence of a transition-metal-containinghydrogenation catalyst in an aqueous solution under conditions such thatthe nitrate group of said water-soluble salt is hydrogenated to form anamine group.

A second aspect of the present invention is a purified salt comprisingAB-PBO monomer ions and phosphate ions wherein at least 99 percent ofthe organic content of the salt is AB-PBO monomer ions.

Processes of the present invention can be used to synthesize monomersalts of the present invention. Monomer salts can be polymerized insolvent acids such as polyphosphoric acid to form AB-PBO polymerswithout a dehydrohalogenation step. AB-PBO polymers can be coagulatedfrom acid dopes to form high strength fibers, films and other shapedarticles.

DETAILED DESCRIPTION OF THE INVENTION

The following terms are used in this application and are defined herefor convenience.

AA-Monomer--A monomer suitable for synthesizing polybenzazole polymers,comprising two electron-deficient carbon groups linked by a divalentorganic moiety (DM) which is inert with respect to all reagents underpolybenzazole polymerization conditions. The electron-deficient carbongroups have the definition and preferred embodiments given herein. Thedivalent organic moiety is preferably alkyl or an aromatic group, asherein defined, is more preferably an aromatic group, and is mostpreferably a six-membered aromatic group. Examples of suitableAA-monomers and references to their synthesis are provided in U.S. Pat.No. 4,533,693 at columns 25-32, Tables 4-6, which is incorporated hereinby reference. Preferred examples of AA-monomers include terephthalicacid, isophthalic acid, bis-(4-benzoic) acid and oxy-bis-(4-benzoicacid) and acid halides thereof.

AB-Monomer--A monomer suitable for synthesizing polybenzazole polymers,comprising an aromatic group, an o-amino-basic moiety bonded to thearomatic group, and an electron-deficient carbon group bonded to thearomatic group. The aromatic group, the electron-deficient carbon groupand the o-amino-basic moiety have the definitions and preferredembodiments given herein. Examples of suitable AB-monomers and processesfor their synthesis are provided in U.S. Pat. No. 4,533,693 at columns33-35, Tables 7-8, which is incorporated herein by reference. Preferredexamples of AB-monomers include 3-amino-4-hydroxybenzoic acid,3-hydroxy-4-aminobenzoic acid, 3-mercapto-4-aminobenzoic acid and theacid halides thereof. AB-monomers are frequently stored as salts ofhydrogen chloride or phosphoric acid, because the free-base of themonomer is unstable susceptible to air oxidation.

Aromatic group (Ar)--any aromatic ring or ring system which can be partof a PBZ polymer. Each aromatic group may individually be heterocyclic,but each is preferably carbocyclic and more preferably hydrocarbyl. Ifan aromatic group is heterocyclic, it is preferably anitrogen-containing heterocycle.

Each aromatic group may comprise a single aromatic ring, a fused ringsystem, or an unfused ring system, containing two aromatic moietieslinked by a bond or a divalent linking moiety which is inert withrespect to PBZ polymerization reagents under PBZ polymerizationconditions. If the aromatic group comprises a divalent linking moiety,that moiety preferably comprises an ether linking moiety, a thioetherlinking moiety, a sulfonyl linking moiety, an alkyl linking moiety, or ahalogenated alkyl linking moiety or known equivalents. The divalentlinking moiety preferably comprises no more than about 6 carbon atoms.Aromatic groups preferably consist essentially of a single ring.

Size of the aromatic group is not critical as long as the aromatic groupis not so big that it prevents further reactions of the moiety in whichit is incorporated. Each aromatic group preferably independentlycomprises no more than about 18 carbon atoms, more preferably no morethan about 12 carbon atoms and most preferably no more than about 6carbon atoms, excluding any divalent linking group and any organicsubstituent on the aromatic group.

Each aromatic group may independently have substituents which are stablein solvent acid and which do not interfere with the polymerization ofmonomers for PBZ synthesis, such as halogen atoms, alkoxy moieties,aryloxy moieties or alkyl moieties. Substituents which comprise organicmoieties preferably comprise no more than about 12 carbon atoms, morepreferably no more than about 6 carbon atoms. Each aromatic grouppreferably has no substituents other than those specified hereinafter.

Azole ring--an oxazole, thiazole or imidazole ring. The carbon atombonded to both the nitrogen atom and the oxygen, sulfur or secondnitrogen atom is the 2-carbon, as depicted in formula III ##STR3##wherein Z is --0--, --S--or --NR--; and R is hydrogen, an aromaticgroup, an aliphatic group or an aliphatic-aromatic group, preferablyhydrogen or an alkyl group, and most preferably hydrogen. R preferablycomprises no more than about 6 carbon atoms, more preferably no morethan about 4 and most preferably no more than about 1. Each azole ringis independently preferably oxazole or thiazole and more preferablyoxazole. In PBZ polymer, the 4 and 5 carbon atoms are ordinarily fusedwith an aromatic group.

Azole-forming moiety--an "o-amino-basic moiety" or "electron-deficientcarbon group," as those terms are hereinafter defined.

o-Amino-basic moiety--a moiety bonded to an aromatic group, whicho-amino-basic moiety contains

(1) a first primary amine group bonded to the aromatic group and

(2) a hydroxy, thiol or primary or secondary amine group bonded to thearomatic group ortho to said primary amine group.

It preferably comprises a hydroxy, thio or primary amine moiety, morepreferably comprises a hydroxy or thiol moiety, and most preferablycomprises a hydroxy moiety. If the o-amino-basic moiety comprises twoamine groups, preferably both are primary amine groups. If theo-amino-basic moiety contains a secondary amine group, the secondaryamine group may comprise an aromatic or an aliphatic group butpreferably comprises an alkyl group. The secondary amine grouppreferably comprises no more than about 6 carbon atoms, more preferablyno more than about 4 carbon atoms and most preferably no more than about1 carbon atom.

BB-Monomer--A monomer suitable for synthesizing polybenzazole polymers,comprising an aromatic group and two o-amino-basic moieties which arebonded to the aromatic group. The aromatic group and the o-amino-basicmoieties have the definitions and preferred embodiments given herein.Examples of suitable BB-monomers and processes for synthesis areprovided in U.S. Pat. No. 4,533,693 at columns 19-24, Tables 1-3, whichis incorporated herein by reference. Examples of preferred BB-monomersinclude 4,6-diaminoresorcinol, 2,5-diaminohydroquinone and1,4-dithio-2,5-diaminobenzene. BB-monomers are frequently stored assalts of hydrogen chloride or phosphoric acid, because the free base ofthe monomer is susceptible to air oxidation.

Electron-deficient carbon group (Q)--any group containing a carbon atomwhich can react in the solvent acid with an o-amino-basic moiety to forman azole ring, such as the groups listed in column 24, lines 59-66 ofthe 4,533,693 patent, which is incorporated herein by reference, andsuch as an orthoester group, an amidate ester group, a trihalomethylgroup or an alkali or alkaline-earth metal carboxylate group. Eachelectron-deficient carbon group is preferably independently a carboxylicacid or acid halide group and more preferably a carboxylic acid group.Halogens in electron-deficient carbon groups are preferablyindependently chlorine, bromine or fluorine and are more preferablychlorine.

Solvent acid--a non-oxidizing liquid acid capable of dissolving PBZpolymers, such as sulfuric acid, methanesulfonic acid, polyphosphoricacid and mixtures thereof. The solvent acid highly preferably either isa dehydrating acid or contains a dehydrating agent such as P₂ O₅ .Examples of preferred solvent acids include polyphosphoric acid andmixtures of methanesulfonic acid and phosphorus pentoxide.Polyphosphoric acid preferably has a P₂ O₅ content by weight of at leastabout 70 percent, more preferably at least about 75 percent andpreferably has a P₂ O₅ content of at most about 90 percent, morepreferably at most about 85 percent. The ratio of methanesulfonic acidto phosphorus pentoxide in mixtures of those compounds is preferably nomore than about 20:1 by weight and no less than about 5:1 by weight. Themost preferred solvent acid is polyphosphoric acid.

Synthesis of AB-PBO Monomer

AB-PBO monomer is preferably synthesized in a three-step process. Thefirst step is the nitration of a hydroxy-ester compound, which contains:

(1) an aromatic group;

(2) a hydroxy group bonded to said aromatic group; and

(3) an ester group having a carboxylate ion linked to said aromaticgroup.

The aromatic group has the description and preferred embodimentspreviously given. The carboxylate ion may be linked to the aromaticgroup by an aliphatic moiety, but is preferably bonded directly to thearomatic group. The hydroxy moiety is preferably not ortho to thecarboxylate ester. It is more preferably para to the carboxylate ester.Examples of suitable hydroxy-ester compounds include the methyl to hexylesters of 4-hydroxybenzoate, 3-hydroxybenzoate,4-(p-hydroxyphenyl)benzoate and 4-(p-hydroxyphenoxy)benzoate. The mostpreferred hydroxy-ester compound is a 4-hydroxybenzoate ester.

Some suitable hydroxy-ester compounds, such as methyl 4-hydroxybenzoate,are commercially available. Other suitable esters can be synthesized byknown reactions such as esterification of an appropriate hydroxybenzoicacid or transesterification of an appropriate hydroxy-benzoate ester.Suitable hydroxy-benzoic acids and related compounds can be synthesizedby obvious variations of known syntheses, such as those described in B.S. Furniss, Vogel's Practical Organic Chemistry--4th ed. 832 (Longman1978); A. H. Blatt et al., 2 Organic Syntheses 343 (J. Wiley & Sons1943): and Fieser, 58 J. Am. Chem. Soc. 1738 (1936), which areincorporated herein by reference.

The hydroxy-ester compound is contacted with a nitrating agent. Thenitration of aromatic compounds is a well-known reaction. The conditionsare familiar to persons of ordinary skill in the art, and are reportedin numerous references, such as G. M. Loudon, Organic Chemistry 590,598, 1283-86 (Addison-Wesley Publishing Co. 1984), which is incorporatedherein by reference. The nitrating agent is preferably nitric acid. Thenitric acid is preferably concentrated, such as about 70 to 71 percentnitric acid. The contact is made in a solvent capable of dissolving thehydroxy-ester compound. The solvent is preferably a halogenatedaliphatic compound and is more preferably methylene chloride. Thesolvent must be inert with respect to nitration and with respect to allreagents under reaction conditions. The contact is made in the presenceof a catalytic amount of strong acid, such as sulfuric acid. Thetemperature is preferably no more than about 25° C., more preferably nomore than about 15° C. and most preferably no more than about 5° C. Itis preferably no less than about -15° C., more preferably no less thanabout -10° C. and most preferably no less than about -5° C.

The nitration product comprises all of the elements of the hydroxy-estercompound, and further comprises a nitro group bonded to the aromaticgroup ortho to the hydroxy group. When the hydroxy group is para to theester moiety, then the nitro groups are primarily all ortho to thehydroxy group. When the hydroxy group is in another position, theproduct may comprise a mixture of isomers which are separated by knowntechniques, such as recrystallization.

In the second step, the nitrated hydroxy-ester is converted to awater-soluble salt and dissolved in an aqueous solvent. The nitratedhydroxy-ester is preferably extracted into an aqueous solution bycontact with an aqueous solvent and with a base which is in sufficientquantities and is selected such that the nitrated hydroxy-ester isconverted into a nitrated hydroxy-benzoate salt which is soluble inwater. The base is preferably an alkali or alkaline-earth metalhydroxide. The base is more preferably an alkali metal hydroxide, suchas lithium, sodium or potassium hydroxide, and is most preferably sodiumhydroxide. The base is preferably dissolved in the aqueous solvent. Theresulting water-soluble salt is preferably formed in yields of at leastabout 90 percent, more preferably at least about 95 percent and mostpreferably at least about 99 percent, based upon the initial amount ofhydroxy-ester compound. The aqueous solution can be used without for thethird step purification or isolation.

In the third step of the synthesis, the product from the second step iscontacted with a hydrogenating agent in the presence of a catalyticamount of transition-metal catalyst in an aqueous solution underconditions such that the nitro group is hydrogenated and an AB-PBOmonomer is formed. Reaction conditions for catalytic hydrogenation ofnitro groups are well-known and are described in many standard texts,such as G. M. Loudon, Organic Chemistry 1197-98 (Addison-WesleyPublishing Co. 1984), which is incorporated herein by reference.

The contact occurs in an aqueous solution, which is preferably thesolution produced in the extraction step. The hydrogenating agent ispreferably molecular hydrogen. The catalyst is preferably a noble metalcatalyst, such as platinum or palladium. It is more preferably apalladium catalyst. The catalyst is preferably supported and morepreferably supported on carbon. Examples of suitable catalyst include5-10 weight percent palladium-on-carbon. The reaction takes place in thepresence of hydrogen. Of course, oxidizing gases such as oxygen must beexcluded from the system. The temperature of the reaction is preferablyat least about 30° C., more preferably at least about 20° C. and mostpreferably at least about 45° C. It is preferably at most about 110° C.,more preferably at most about 95° C. and most preferably at most about65° C.

The product of the hydrogenation step is an AB-PBO monomer in which theelectron-deficient carbon group is a carboxylate salt. The positions ofthe amine, hydroxy and carboxylate groups in the monomer are determinedby the position which they and their precursors held in theintermediates used to make the monomer. The monomer is dissolved in anaqueous solution, and is highly susceptible to air oxidation in itsfree-base state.

The AB-PBO monomer may be precipitated from the aqueous solution bycontacting it with a non-oxidizing protic avoid to convert thecarboxylate salt moiety into a carboxylic acid moiety. The non-oxidizingprotic acid is preferably chosen and in sufficient quantities toprotonate the o-amino-hydroxy moiety of the monomer, in order tostabilize the monomer against air oxidation. The non-oxidizing proticacid is preferably a hydrogen halide or phosphoric acid, more preferablyhydrochloric acid or phosphoric acid, and most preferably phosphoricacid. The monomer precipitates as an acid salt of the non-oxidizingprotic acid. The monomer should not be exposed to air or other oxidizingmedia until it has been contacted with an acid to protonate theo-amino-hydroxy moiety.

The precipitated AB-PBO monomer phosphate salt can be purified byrecrystallization from an aqueous phosphoric acid solution. The solutionmay contain a small amount of reducing agent, such as tin (II) chloride,to reduce any oxidized impurities in the monomer. The solution is heatedto any temperature sufficient to cause essentially all of the monomerphosphate salt to dissolve. The temperature is preferably at least about80° C., more preferably at least about 90° C. and most preferablygreater than 100° C. The solution is then cooled to a temperaturesufficient to precipitate a substantial portion of monomer phosphate.The temperature is preferably at most about 10° C., more preferably atmost about 5° C. and most preferably at most about 0° C.

The recrystallized monomer is preferably washed with a volatile organicnon-solvent which forms an azeotrope with water. The non-solvent ispreferably an alcohol having from 1 to 6 carbon atoms, and is morepreferably ethanol. The monomer is preferably dried to removeessentially all water and free phosphoric acid.

The processes described above preferably produce at least about 75percent yield of AB-PBO monomer based upon the beginning hydroxy-estercompound, and more preferably produce at least about 80 percent yield.The purity of AB-PBO monomer phosphate salt, as a weight percentage ofthe organic content which is AB-PBO monomer ion, is preferably at leastabout 99 percent, more preferably at least about 99.5 percent and mostpreferably at least about 99.9 percent by weight.

The AB-PBO monomer phosphate salt comprises ions of AB-PBO monomer andphosphoric acid. The AB-PBO monomer has the description previouslygiven. The aromatic group has the description and preferred embodimentsof the aromatic group in the initial hydroxy-ester compound. Theelectron-deficient carbon group has the meaning and preferredembodiments previously given. It is most preferably carboxylic acid. Theelectron-deficient carbon group is preferably para to the amine moietyor the hydroxy moiety, and most preferably to the hydroxy moiety. TheAB-PBO monomer ion is most preferably an ion of 3-amino-4-hydroxybenzoicacid.

The phosphate ion may, in some cases, be a condensed phosphate, such aspyrophosphate, but is preferably a single phosphate. A single phosphateion may have two or three AB-PBO monomer ions associated with it, butthe average number of phosphate ions associated with each AB-PBO ion ispreferably about 1:1.

AB-PBO monomer phosphate ions of the present invention may bepolymerized in non-oxidizing solvent acid according to known processes.The solvent avoid is preferably polyphosphoric acid containing at least80 percent P₂ O₅. When the electron-deficient carbon group does notcontain halogen, no dehydrohalogenation step is necessary. Preferredconditions are the polymerization conditions discussed in U.S. Pat. Nos.4,772,678; 4,703,103; 4,533,692; 4,533,724; 4,533,693; 4,359,567; and4,578,432 and 11 Ency. Poly. Sci. & Eng., supra, 601, which arepreviously incorporated herein by reference. The temperature ispreferably started at no higher than about 60° C., and raisedperiodically throughout the reaction. The maximum temperature during thereaction is preferably at least about 70° C., more preferably at leastabout 100° C., more highly preferably at least about 150° C. and mostpreferably at least about 190° C. The maximum temperature must be lessthan the decomposition point of reagents and products, and is preferablyat most about 230° C., more preferably at most about 210° C. Thereaction should be carried out under a non-oxidizing atmosphere and withvigorous agitation. Other conditions may be found in other literaturereferences.

AB-PBO polymer produced by this process preferably has a molecularweight corresponding to an inherent viscosity in methanesulfonic acid at25° C. and about 0.05 g/dL of at least about 7 dL/g, more preferably atleast about 10 dL/g, more highly preferably at least about 12 dL/g andmost preferably at least about 14 dL/g. AB-PBO monomer phosphate saltsmay also be used in other manner known for hydrochloride salts, such asby polymerizing with AA- and BB-PBZ monomers to form random or blockcopolymers or by polymerizing with AB-PBT or -PBI monomers to formrandom copolymers.

AB-PBO polymers can be extruded into fibers and films useful for makingcomposites and laminates according to the processes described in 11Ency. Poly. Sci. & Eng., supra, at 625-31, which is incorporated hereinby reference.

WORKING EXAMPLES

The following examples are for illustrative purposes only and are notintended to limit the scope of either the specification or the claims.All parts and percentages are by weight unless otherwise stated.

EXAMPLE 1 Synthesis and Precipitation of AB-PBO Monomer Phosphate Salt

A 152.0-g (1.0 mole) quantity of methyl 4-hydroxybenzoate is dissolvedin one liter of dichloromethane. The solution is cooled to 0° C. and 100ml of 98 percent sulfuric acid is added. With vigorous stirring 80 ml of71 percent nitric acid is added dropwise such that the temperature ofthe mixture does not exceed 5° C. The mixture is stirred an additionalhour at 5° C. The mixture is diluted with 150 ml of water and theaqueous phase is separated.

The organic phase is diluted with 500 ml of dichloromethane, washed with200 ml of water, and extracted with three 500-ml portions of aqueoussodium hydroxide containing 45 g (1.1 moles) of sodium hydroxide. Thethree aqueous extracts, containing sodium 4-hydroxy-3-nitrobenzoate arecombined.

A 500-ml portion of the extract containing approximately 0.33 mole ofsodium 4-hydroxy-3-nitrobenzoate is sealed in a one-liter Hastelloy Cautoclave with 5.0 g of 5 percent palladium-on-carbon. The reactor ispurged with nitrogen and charged to 400 psig with hydrogen gas. Thereactor is heated to 45° C. and maintained at 300 psig to 400 psighydrogen pressure until hydrogen uptake is completed. The reactor ispurged with nitrogen, the catalyst is filtered and the solution isacidified with 600 ml of 85 percent phosphoric acid. The solution iscooled to 0° C and crude 3-amino-4-hydroxybenzoic acid hydrophosphatesalt is filtered.

The crude salt is added to a mixture of 600 ml of 85 percent phosphoricacid, 1.0 g of SnCl₂ 2H₂ O dissolved in 50 ml of 35 percent HCl, and 100ml of water. The slurry is heated to 150° C. and water is added insufficient quantity to dissolve all solid material. A 5-g quantity ofactivated carbon is added and the solution is maintained at 150° C. for10 minutes. The carbon is filtered, and the filtrate is cooled to 0° C.The resulting crystals are filtered, washed in cold n-propanol and driedunder nitrogen gas. The recovered product contains 71 g (80 percentyield) of 3-amino-4-hydroxybenzoic acid hydrophosphate salt having onewater of hydration.

EXAMPLE 2 Synthesis of AB-PBO Phosphate Monomer with Nickel Reduction

The process of Example 1 is repeated, except that the reduction iscarried out using 2.5 g of nickel catalyst at a temperature of 95° C.The yield is approximately the same.

EXAMPLE 3 Polymerization of AB-PBO Monomer Phosphate Salt

Under nitrogen atmosphere, 15.0 g of AB-PBO monomer phosphate salt fromExample 1 and 6.43 g of polyphosphoric acid containing 77 weight percentP₂ O₅ are agitated at 95° C. for 1.3 hours. No foaming is observed. A17.6-g quantity of P₂ O₅ is added, and agitation at 95° C. is continuedfor 4 hours. A 4.37-g quantity of polyphosphoric acid is added, andstirring is continued at 95° C. for 18 hours. The pressure is reduced to160 Torr, and stirring at 95° C. is continued for 24 hours. Thetemperature is raised to 190° C. with stirring at 160 Torr for 24 hours.

The resulting dope is extruded according to known methods to form afiber. AB-PBO polymer is coagulated from a sample of dope, washed withwater, dried, ground, rewashed and redried. The polymer has an inherentviscosity of 14.5 dL/g in methanesulfonic acid at 25° C. and 0.0528 g/dLconcentration.

What is claimed is:
 1. A process for synthesizing an AB-PBO monomer,said process comprising the steps of:(1) contacting a hydroxy-estercompound comprising:(a) an aromatic group: (b) a hydroxy group bonded tosaid aromatic group; and (c) an ester moiety having a carboxylate moietylinked to said aromatic group with a nitrating agent under conditionssuch that the aromatic group is nitrated in a position ortho to thehydroxy group, in an organic solvent, which is inert with respect to allreagents under reaction conditions: (2) converting the nitratedhydroxy-ester of step (1) to a water-soluble nitrated hydroxy-benzoatesalt and dissolving said water-soluble salt in an aqueous solvent: and(3) contacting the water-soluble salt product of step (2) with ahydrogenating agent in the presence of a transition-metal-containinghydrogenation catalyst in an aqueous solution under conditions such thatthe nitrate group of said water-soluble salt is hydrogenated to form anamine group.
 2. The process of claim 1 wherein:(a) the aromatic group inthe hydroxy-ester compound comprises no more than about 12 carbon atoms:(b) the nitrating agent of step 1 is nitric acid; (c) step (2) comprisesextracting the nitrated hydroxy-ester of step (1) by contacting saidnitrated hydroxy-ester in an organic solvent with an aqueous solvent andwith a base sufficient to convert said nitrated hydroxy-ester into awater-soluble salt: and (d) the hydrogenating agent of step (3) ismolecular hydrogen.
 3. The process of claim 2 wherein the aromatic groupin the hydroxy-ester compound comprises no more than about 6 carbonatoms and the base of step (2) is an alkali metal hydroxide oralkaline-earth metal hydroxide.
 4. The process of claim 3 wherein thearomatic group in the hydroxy-ester compound is a carbocyclic group. 5.The process of claim 4 wherein the transition-metal-containinghydrogenation catalyst of step (3) contains nickel.
 6. The process ofclaim 4 wherein the transition-metal-containing catalyst of step (3)contains a noble metal.
 7. The process of claim 6 wherein thetransition-metal-containing catalyst of step (3) contains palladium. 8.The process of claim 7 wherein the transition-metal-containing catalystof step (3) is a palladium-on-carbon catalyst.
 9. The process of claim 8wherein the yield of AB-PBO monomer is at least about 80 percent. 10.The process of claim 4 wherein the base of step (2) is an alkali metalhydroxide.
 11. The process of claim 10 wherein the base of step (2) issodium or potassium hydroxide.
 12. The process of claim 4 which furthercomprises the step of:(4) contacting an aqueous solution containing theAB-PBO monomer produced in step (3), with a non-oxidizing protic acidchosen such that and in sufficient quantities such that the carboxylatemoiety of the monomer is converted to a carboxylic acid moiety and theAB-PBO monomer precipitates as an acid salt.
 13. The process of claim 12wherein the non-oxidizing protic acid of step 4 is hydrochloric acid orphosphoric acid.
 14. The process of claim 13 wherein the non-oxidizingprotic acid of step 4 is a phosphoric acid.
 15. The process of claim 14wherein the AB-PBO monomer phosphate salt is recovered with a yield ofat least about 75 percent by mole, based upon the initial hydroxy-ester,and with a purity of at least about 99 percent by weight, based upon theratio of AB-monomer ion content to total organic content in the product.16. The process of claim 15 wherein the AB-PBO monomer phosphate salt isa phosphate salt of 3-amino-4-hydroxybenzoic acid.
 17. The process ofclaim 4 wherein the hydroxy moiety of the hydroxy-ester compound is inpara position with respect to the ester moiety.
 18. The process of claim17 wherein the AB-PBO monomer which is recovered is3-amino-4-hydroxybenzoic acid.
 19. The process of claim 18 wherein3-amino-4-hydroxybenzoic acid is recovered with a yield of at leastabout 75 percent by mole, based upon the initial hydroxy-ester.
 20. Theprocess of claim 1 wherein the hydroxy-ester compound is a4-hydroxy-benzoate ester.
 21. The process of claim 20 wherein:(a) the4-hydroxy-benzoate ester is a 1-6 carbon alkyl ester. (b) the nitratingagent of step 1 is nitric acid; (c) step (2) comprises extracting thenitrated hydroxy-ester of step (1) by contacting said nitratedhydroxy-ester in an organic solvent with an aqueous solvent and with abase sufficient to convert said nitrated hydroxy-ester into awater-soluble salt; and (d) the hydrogenating agent of step (3) ismolecular hydrogen.
 22. The process of claim 21 wherein the base of step(2) is an alkali metal hydroxide or alkaline-earth metal hydroxide. 23.The process of claim 22 wherein the transition-metal-containinghydrogenation catalyst of step (3) contains nickel.
 24. The process ofclaim 22 wherein the transition-metal-containing catalyst of step (3)contains a noble metal.
 25. The process of claim 24 wherein thetransition-metal-containing catalyst of step (3) contains palladium. 26.The process of claim 25 wherein the transition-metal-containing catalystof step (3) is a palladium-on-carbon catalyst.
 27. The process of claim26 wherein the yield of AB-PBO monomer is at least about 80 percent. 28.The process of claim 23 wherein the base of step (2) is an alkali metalhydroxide.
 29. The process of claim 28 wherein the base of step (2) issodium or potassium hydroxide.
 30. The process of claim 22 which furthercomprises the step of:(1) contacting an aqueous solution containing theAB-PBO monomer produced in step (3), with a non-oxidizing protic acidchosen such that and in sufficient quantities such that the carboxylatemoiety of the monomer is converted to a carboxylic acid moiety and theAB-PBO monomer precipitates as an acid salt.
 31. The process of claim 30wherein the non-oxidizing protic acid of step 4 is hydrochloric acid orphosphoric acid.
 32. The process of claim 31 wherein the non-oxidizingprotic acid of step 4 is a phosphoric acid.
 33. The process of claim 32wherein the AB-PBO monomer phosphate salt is recovered with a yield ofat least about 75 percent by mole, based upon the initial hydroxy-ester,and with a purity of at least about 99 percent by weight, based upon theratio of AB-monomer ion content to total organic content in the product.34. The process of claim 2 wherein the hydroxy-ester compound is amethyl, ethyl, propyl, butyl, pentyl or hexyl ester of4-hydroxybenzoate, 3-hydroxybenzoate, 4-(p-hydroxyphenyl)benzoate or4-(p-hydroxyphenoxy)benzoate.